1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for controlling pressure and mix ratio in gas delivery. More specifically, embodiments of the invention generally relate to a method and apparatus for controlling pressure and mix ratio of gas provided to a semiconductor processing chamber.
2. Description of the Related Art
Processing gases are widely used in semiconductor processing. Processing gases may be provided as reactant gas, carrier gas or purge gas to process a front side of a substrate. Gases may also be provided between a substrate and a substrate support in a semiconductor processing chamber to maintain a precise and uniform of substrate temperature.
Pressure and mix ratio of the processing gas are important process control attributes to many semiconductor processes. Pressure and mix ratio are conventionally controlled separately.
FIG. 1 depicts a simplified schematic of a conventional semiconductor processing chamber 100 having a gas delivery system 120 shown providing processing gas to a processing volume 103 in the processing chamber 100. The processing volume 103 defined by a chamber body 101 is configured to process a substrate 104 therein. The processing volume 103 is in selective fluid communication with a vacuum pump 105 via a throttle valve 107. The vacuum pump 105 maybe connected to a rough pump 106. The processing chamber 100 may be configured to perform chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), etching or other processing technique. Process gas delivery systems, pumping systems and the like for controlling processes performed within the processing chamber are well-known and have been omitted for the sake of brevity.
The gas delivery system 120 is configured to mix and deliver two or more gases to the processing chamber 100 via a gas line 131. A shut off valve 129 is positioned on the gas line 131. As shown in FIG. 1, the gas delivery system 120 includes a first gas supply and a second gas supply line parallel to one another. The first gas supply line include a gas source 121 connected to the gas line 131 via a shut off valve 123 and a control valve 127. The second gas supply line includes a gas source 122 connected to the gas line 131 via a shut off valve 124 and a control valve 127.
During process, gases from the gas sources 121, 122 come through control valves 127, 128 respectively, are mixed together in the gas line 131 and are delivered to the processing volume 103. The ratio of each gas in the gas mixture is controlled by a ratio controller 130. The ratio controller 130 adjusts the control valves 127, 128 to obtain desired mix ratio in the gas delivered to the processing chamber 100.
Chamber pressure is controlled separately from the mix ratio. A pressure control unit 110 may be used to obtain a desired pressure the chamber. The pressure control unit 110 may adjust the chamber pressure by adjusting the throttle valve 107 according to feedback from a chamber pressure gauge 109. Chamber pressure is a result of flow rate from the gas line 131 and the status of the throttle valve 107.
The conventional chamber pressure and mix ratio control described above has a relatively slow responding time to achieve a target chamber pressure and/or a target mix ratio. For example, when adjusting chamber pressure from a lower pressure to a higher pressure, the pressure control unit 108 decreases opening of the throttle valve 107 and wait for more gas flowing in from the gas line 131 to increases the chamber pressure. On the one hand, it may take a few seconds before the chamber pressure reaches the desired value, which is long relative to a typical semiconductor processing step. On the other hand, it takes relatively long time to adjust a mix ratio in the process volume 103 too. For example, when adjusting a mix ratio from 50% gas A-50% gas B to 70% gas A-30% gas B, the mix ratio controller 130 can adjust the control valves 125, 126 to change the flow rates supplied to the gas line 131. However, existing gas in the processing volume 103 will have to be pumped out before mix ratio reaches desired valve in the processing volume 103.
Additionally, the above described chamber pressure and mix ratio control requires two sets of controllers which increase total cost.
Therefore, there is a need for an improved method and apparatus for controlling chamber pressure and gas mix ratio in a semiconductor processing system.